it provides a first-hand insight into context for the loss,
such as causal connections, geographical associations, trade
patterns and many other areas, providing a microcosm of our
maritime heritage and maritime
history.

Types of wreck diving

In his seminal work on the subject, The Advanced Wreck Diving
Handbook, Gary Gentile sub-divides
wreck diving into three categories:

Non-penetration wreck diving is the least hazardous form of wreck
diving, although divers still need to be aware of the entanglement
risks presented by fishing nets and fishing lines which may be
snagged to the wreck (wrecks are often popular fishing sites), and
the underlying terrain may present greater risk of sharp
edges.

Penetration within the light zone presents greater hazards due to
overhead and greater proximity of the wreck's structure, but
because of the proximity of a visible exit point, and some amount
of external light, those hazards are more manageable. However,
there is clearly a much greater risk of entanglement and siltout
inside of the structure, as well as the requirement to move
laterally to a defined exit point before one can surface in the
event of an emergency.

Full penetration involves the greatest level of risks, including
the risk of getting lost within the structure, the risk of complete
darkness in the event of multiple light failures, and the inability
to escape unassisted in the event of a disruption to air
supply.

These categorisations broadly coincides with the traditional
division between "recreational"
wreck diving (taught as a speciality course by recreational diver
training agency which is normally expressed to be limited to the
"light zone" and/or 100 cumulative feet of depth plus penetration)
and "technical" wreck diving
(taught as a stand alone course by technical diver training
agencies).

Wreck diver training and safety

Wrecks may pose a variety of unique hazards to divers. Wrecks are
often snagged by fishing lines or nets and the structure may be
fragile and break without notice. Penetration diving, where the diver
enters a shipwreck is an advanced skill requiring special training
and equipment. Many attractive or well preserved wrecks are in
deeper water requiring deep diving
precautions. It is essential that at least one cutting device be
carried in the event that the diver is entangled with fishing lines
or ropes and to have a spare light source in case the primary light
fails. If penetrating a wreck, a guideline tied off before entering
a wreck and run out inside the wreck is advisable. A guideline
helps a wreck diver in finding the way out easier in case of low
visibility due to stirred up sediments. For penetration diving, a
greater reserve of breathing gas should be allowed for, to ensure
there is sufficient to get out of the wreck. Most wreck divers use
a minimum of the rule-of-thirds for gas management.
This allows for 1/3 of the gas down and into the wreck, 1/3 for
exit and ascent and 1/3 reserve. In addition, because of the
potential fragility of the wreck, the likelihood of disturbing
sediments or disturbing the many marine animals that take advantage
of the artificial habitat offered by the wreck, extra care is
required when moving and finning. Many divers are taught to use
alternative finning methods such as frog
kick when inside a wreck. Perfect buoyancy control is a must
for diving in the environment of a
wreck.[[Image:shipwreckporthole.jpg|frame|Diver with porthole
recovered from a shipwreck in New York's Wreck Alley: Shipwreck Expo]]

Many diver training
organizations provide specialist wreck diver training courses, such as SDI, and PADI
Wreck Diver, which divers are advised to take before wreck diving.
Such courses typically teach skills such as air management and the
proper use of guidelines and reels. Most recreational diving
organizations teach divers only to penetrate to limit of the "light
zone" or a maximum aggregate surface distance (depth + penetration)
of 100 feet (whichever is the lesser). Other technical diving organizations, such as
IANTD, TDI, and ANDI teach advanced
wreck courses, that emphasize a higher level of training,
experience and equipment and prepare divers for deeper levels of
wreck penetration. The Nautical Archaeology Society in
the UK, teaches awareness of underwater cultural heritage issues as well as
practical diver and archaeological skills. Other
organizations, such as the Artificial Reef Society of British
Columbia (ARSBC) deliberately create artificial reefs to provide features for
divers to explore, as well as substrates for marine life to thrive
upon.

Penetration diving

In technical penetration diving, there are broadly two approaches.
The conventional approach involves the use of continuous guidelines
laid from a wreck reel, tied just outside of the entrance point,
just inside the entrance point, and at regular intervals inside (to
mitigate the risk of a cut line, or a "line trap"). In deeper
penetrations, two reels are used, so that in the event of a total
loss of visibility where the diver loses contact with the primary
line or the primary line gets cut, the secondary line can be
anchored and then used as a reference point to sweep for the
primary line.

An alternative approach, popularised by deep wreck divers in the
American Northeast, is referred to as "progressive penetration".
Progressive penetration eschews the use of reels, but the diver
makes several successive penetrations, each successively deeper
than the last, memorising the layout for both the inward and
outward journeys. As a diving technique, progressive penetration is
not taught by any of the mainstream diver training agencies.

Divers engaging in penetration diving are conventionally taught to
carry three lights - a primary light and two backup lights -
thereby virtually eliminating the risk of completely losing light
inside the wreck. Nonetheless, total loss of visibility due to a
silt-out remains a risk.

Deep diving and wreck diving

Diver returning from a 600ft wreck
dive

Wrecks in shallower waters tend to deteriorate faster than wrecks
in deeper water due to higher biological activity. Accordingly,
many of the older and larger shipwrecks that tend to offer full
penetration dives tend to be deeper dives. This can present
additional complications; if a wreck dive is intended to be a
decompression dive, then the
diver will normally carry decompression gases in sidemount cylinders. However, it is difficult to
penetrate many wrecks with sidemount cylinders, requiring divers to
either use a different configuration, or leave their decompression
gases outside the wreck prior to penetration. This creates the
possibility of a diver being unable to relocate their decompression
gases if they exit the wreck at a different point from which they
enter it.

Protection of wrecks

In many countries, wrecks are legally protected from unauthorized
salvage or desecration.

Protection
of Military Remains Act 1986 : all military aircraft and 16
designated ships are considered war graves
that can only be dived with a license. Other non-designated ships
may be dived providing the divers do not enter, disturb or remove
artifacts

In
Greece, during the year 2003 the Greek Government
(ministry of culture), issued a Ministerial Order classifying "any
wreck of ship or aeroplane, sunk for longer than 50 years from the
present" as Cultural Assets / Monuments, setting also a protection
zone of 300 meters around them. Terms and conditions for
visiting any monument in Greece are set by the Ministry of Culture
in Greece.

For
technical divers there are fewer wrecks that have attracted
widespread popularity, although for years the SS Andrea
Doria was regared as the "Mount Everest" of wrecks to challenge the diver. However,
since the popularisation of using trimix as a breathing gas, technical
divers now routinely dive much deeper and more challenging wrecks,
and the Andrea Doria is argued by some to now be a good
training wreck for trimix divers.

References

A line trap refers to the situation where a line is laid
between two points, but when taut, the line stretches through a
narrower opening which the diver cannot swim through. With
visibility this is not a problem as the diver can retrace his
original route; but with a complete loss of visibility the diver
would be unable to follow the line by touch alone back to the exit
point.

In 1992 two divers, Chris and Chrissy Rouse, died of
decompression sickness after becoming
trapped in the wreck whilst diving German
submarine U-869 off the New Jersey coast, and then being unable
to relocate their decompression gases after they escaped. The
incident became famous after being chronicled in various books,
including the New York Times best-selling book,
Shadow
Divers.

In his book, Wreck Diving Adventures, author
Gary Gentile
says: "I cannot state too often, the Great Lakes have the finest
wreck diving the world."

An excellent discussion of this is to be found in Kevin
McMurray's book, Dark Descent, ISBN 0-7434-0063-1. He
discusses how some older divers react poorly to use of the
Andrea Doria as a training wreck, perceiving it as
diminishing their achievements.